President Obama and South Korea President Park Geun-Hye walk off stage at the end of a news conference May 7 at the White House. (Pablo Martinez Monsivais/AP)

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WASHINGTON — Projecting a united front, President Obama and South Korea's new leader warned North Korea on Tuesday against further nuclear provocations, with Obama declaring that the days when Pyongyang could "create a crisis and elicit concessions" were over.

Obama also disputed the notion that his cautious response to reported chemical weapons use in Syria — a move he had said would cross a "red line" — could embolden North Korea's unpredictable young leader and other U.S. foes.

"Whether it's bin Laden or Gadhafi, if we say we're taking a position, I would think at this point the international community has a pretty good sense that we typically follow through on our commitments," Obama said, referring to the al-Qaida commander Osama bin Laden and former Libyan leader Moammar Gadhafi, both of whom were killed during Obama's watch.

Tuesday's meetings between Obama and South Korean President Park Geun-hye followed months of increased tensions on the Korean Peninsula. North Korea conducted an underground atomic test in February and had appeared ready for another. New U.S. intelligence assessments also showed for the first time that North Korea may have the know-how to launch a nuclear-armed missile, though American officials say Pyongyang still appears to lack the capability to carry out an attack.

Ahead of Tuesday's talks, the North appeared to send mixed messages. U.S. officials said Pyongyang removed from a launch pad a set of medium-range ballistic missiles that had been readied for possible test-firing. But North Korea also warned the U.S. and South Korea that it would retaliate if joint military exercise between the two allies resulted in any shells landing on its territory.

Speaking at a joint news conference at the White House, Obama and Park warned Pyongyang of unspecified consequences if it pressed ahead with provocative actions, with Obama vowing to protect the U.S. and its allies using both "conventional and nuclear forces."

Still, in keeping with their countries' long-standing policies, the two leaders left open the possibility of direct negotiations should the North signal its readiness to end its nuclear pursuits or take other meaningful actions.

"Should North Korea choose the path to becoming a responsible member of the community of nations, we are willing to provide assistance, together with the international community," Park said.

Analysts see some of North Korea's recent bluster as an attempt by the country's new leader, Kim Jong Un, to establish himself as a power player, both within his own country and in the international community. Obama said he knew little about Kim personally and has never spoken to him, but added that his actions were leading him down a dead end.

"There's going to have to be changes in behavior," Obama said. "We have an expression in English, 'Don't worry about what I say, just watch what I do.'"

North Korea ratcheted up its provocations this year after the U.N. Security Council tightened sanctions in response to the February nuclear test, its third since 2006. Pyongyang claims to have scrapped the 1953 Korean War armistice and has threatened nuclear strikes on the U.S., prompting Washington to bolster missile defenses.

While another nuclear test had seemed likely, a pair of launch-ready missiles has been removed from a launch pad, according to two U.S. officials. The officials spoke on condition of anonymity because they were not authorized to publicly discuss a matter involving sensitive U.S. intelligence.

Park's visit was also focused on building a rapport with Obama, who had a close relationship with her predecessor, Lee Myung-bak.

Lee took a hard line on relations with Pyongyang, cutting aid to the impoverished nation. While his approach had Obama's firm backing, public frustration in the South has mounted over the North's continued weapons tests and other provocative actions, including attacks in 2010 that left dozens of South Koreans dead.

In a change of tone, Park, although a conservative, has advocated trying to build trust with Pyongyang through aid shipments and large-scale economic initiatives if there's progress on the nuclear issue, even as she and South Korea's military promise to respond forcefully to any attack from the North.

Obama made clear Tuesday that there continues to be no daylight between the White House and the new South Korean leader.

"If Pyongyang thought its recent threats would drive a wedge between South Korea and the United States or somehow garner the North international respect, today is further evidence that North Korea has failed again," he said.

Park arrived at the White House with a color guard lining the driveway from Pennsylvania Avenue. Her Oval Office meeting, working lunch and joint news conference with Obama will be followed Wednesday by an address to a joint meeting of Congress.

"My daughters have taught me a pretty good 'Gangnam Style,'" Obama joked, a reference to the hit dance song by South Korean singer PSY that has become YouTube's most watched video with 1.5 billion views since its release last summer.

Associated Press writers Matthew Pennington and Robert Burns contributed to this report.